The invention relates generally to methane and, more particularly, to a process and arrangement for the production of a methane-containing gas.
Gases having a high methane content may, for instance, find an application as exchange gases for natural gas. Recently, numerous proposals for the production of natural gas exchange gas have become known. The starting materials which may be used include coke-oven gas and liquid, low-boiling hydrocarbons such as, for example, benzine. In addition, however, coal and/or heavy oil may also be used as starting materials.
If the two last-mentioned materials are used, then it is advantageous when the starting material is initially subjected to a partial oxidation (gasification). This may be accomplished by known processes such as the Koppers-Totzek process, the Shell process or the Texaco process. Which process is most favorably used depends primarily on the type and character of the starting material to be gasified. Likewise, the composition of the gas obtained from the partial oxidation is dependent upon the starting material used. This is clarified by the following Table where a comparison is given of the composition of the gas of partial oxidation obtained from the gasification of coal and that of the gas of partial oxidation obtained from the gasification of heavy oil:
TABLE ______________________________________ Starting Material Gas Composition (percent by volume) Coal Heavy Oil ______________________________________ CO.sub.2 + H.sub.2 S and other sulfur compounds 10.0 5.0 CO 58.5 47.0 H.sub.2 30.0 46.5 CH.sub.4 0.5 0.5 N.sub.2 + Ar 1.0 1.0 ______________________________________
The partial oxidation gas is then usually subjected to a desulfurization during which sulfur compounds contained in the gas are removed therefrom in accordance with known processes. Following the desulfurization, it has heretofore been the practice to catalytically convert a portion of the carbon monoxide contained in the partial oxidation gas, which catalytic conversion proceeds according to the following equation in known manner: EQU CO + H.sub.2 O .fwdarw. H.sub.2 + CO.sub.2 ( 1)
the carbon dioxide formed during this reaction may then, by known means, be removed from the process. For the desulfurization and conversion, there are now available the most diverse, reliably proven possibilities and combinations. For instance, the conversion may take place prior to the desulfurization since there are presently available both sulfur-resistant and sulfur-susceptible conversion catalysts.
Subsequent to the desulfurization and conversion, it has heretofore been the practice to subject the partial oxidation gas, which now has a more or less high carbon monoxide content and which has been more or less freed of carbon dioxide, to a methanization reaction which proceeds catalytically essentially according to the following equation: EQU CO + 3H.sub.2 .fwdarw. CH.sub.4 + H.sub.2 O (2)
simultaneously, the carbon dioxide still remaining in the gas is methanized according to the following equation: EQU CO.sub.2 + 4H.sub.2 .fwdarw. CH.sub.4 + 2H.sub.2 O (3)
the known procedures outlined above have certain disadvantages, however. Thus, on the one hand, large quantities of water vapor are required for the conversion reaction since, aside from the water vapor required for the conversion reaction itself, it is also necessary to provide large quantities of equilibrium water vapor. On the other hand, during the methanization reaction which follows the conversion reaction, there is produced water vapor which has heretofore remained unused and which it has been necessary to eventually condense out of the gas.
A process for the production of normal municipal gas is known from the German patent 1,085,287 wherein a gas of synthesis is simultaneously methanized and converted. This process operates with special sulfide catalysts obtained from the elements of the sixth group of the Periodic System and, as a result, the gas which is to undergo reaction must contain a minimum of 100 to 1000 milligrams of sulfur per Nm.sup.3 of gas in the form of sulfur compounds. This process, however, does not achieve the objective of reducing the quantities of water vapor required. In contrast, it is unconditionally required that adequate quantities of additional water vapor be utilized. This is necessary so that, on the one hand, the equilibrium of the methanization reaction will not be displaced too far to the right, which would result in an undesired increase in the calorific value of the municipal gas produced, and so that, on the other hand, adequate quantities of water vapor are available for the conversion reaction.